Accuracy of precipitation measurements, instrument ... · International Comparison of National...
Transcript of Accuracy of precipitation measurements, instrument ... · International Comparison of National...
University of Genova - DICCA
Dept of Civil, Chemical and Environmental Engineering
WMO/CIMO Lead Centre “B. Castelli”
on Precipitation Intensity
Luca G. Lanza
Mattia Stagnaro
Arianna CauteruccioTokyo,
22 M
arc
h 2
018
Accuracy of precipitation measurements,
instrument calibration and techniques
for data correction and interpretation
JMA/WMO Workshop on Quality Management of Surface Observations
RA II WIGOS Project
Tokyo, Japan, 19-23 March 2018
WMO
INTRODUCTION
from 0.02 to 2000 mmh-1
from 0.02 to 0.2 mmh-1 rep. as trace
Time resolution : 1 minute
Max acceptable error for RI :
• from 0.2 to 2 mmh-1: 0.1 mmh-1
• from 2 to 2000 mmh-1: 5 %
Applications of the “rain intensity” variable:
- Meteo-hydrological warnings
- “coupling” of meteorological and hydrological models
- Flood forecasting, protection and mitigation
- Urban hydrology, engineering design
- etc.
Measurement of Rainfall Intensity (RI)
(lack of knowledge, expertise, standardization,
recommendations, instruments, etc.)
WMO Expert Meeting on Rainfall Intensity Measurements
Bratislava (Slovakia), April 2001
Intercomparison of measurement instruments
I° phase: Laboratory tests (counting errors in
controlled conditions)
II° phase: Field Intercomparison (catching
errors in operational conditions)
. International Comparison of National Precipitation Gauges with a Reference Pit Gauge
(Sevruk et al., 1984).
. WMO Solid Precipitation Measurement Intercomparison (Goodison et al., 1998).
(precipitation intensity first time studied in meteorological evaluations)
. WMO Intercomparison of Present Weather Sensors/Systems (Leroy et al., 1998).
only for qualitative information (light, moderate, intense)
focused on cumulative (total) precipitation
low precipitation intensity (snow)
combined effect of counting and catching errors
catching-type gauges only
Catching errors = Errors due to the atmospheric conditions at the collector, as well as to the
wetting, splashing and evaporation issues. Indicate the capability of the instrument to collect the
volume of water corresponding to the definition of precipitation at the ground, i.e. the amount of
water falling through the horizontal projection of the collector area.
Counting errors = Related to the capacity of the instrument to correctly “sense” the amount
of water actually collected by the instrument. These errors occur for both the catching and non
catching types of gauges, even if in the latter case their quantification is really difficult, and can
hardly be performed in laboratory conditions.
Previous WMO Intercomparison Experiences
Symbol Type of error MagnitudeMeteorological
influencing factors
Instrumental influencing
factors
k
Losses due to the deformation
of the airflow above the
instrument collector
2-10%
(10-50%
for snow)
Wind velocity and
precipitation
microstructure
Shape, area and height of
the collector
Pg1
+
Pg2
Losses due to wetting of
internal walls of the collector
and the mechanics of the
instrument
2-10%
Rainfall intensity, type of
precipitation, tipping
bucket movements
Shape, area and height of
the collector, age and
materials of both the
collector and the
measuring unit
Pg3 Evaporation losses 0-4%
Type of precipitation, air
temperature and wind
velocity between the end
of precipitation and its
measurement
Surfaces of the collector
and the measuring unit
Pg4 Splashing of drops 1-2%Rainfall intensity and wind
velocity
Shape and height of the
collector, type of
installation
i
gigC PPkP ][- PC: corrected value;
- Pg: precipitation measured by the instrument;
- Pgi: correction terns for various error sources;
- k: correction coefficient for wind effects.[Sevruk, 1979]
Correction of precipitation measurement errors
Liquid
precip.
Solid
precip. always
k > 1
(underestimation)
orographic
vs.
convective
Totalizer TBR
Scale ?
WMOWMO
Further catching errors …
Most of the uncertainties due to catching problems have a
limited impact on the measurement of heavy rainfall rates,
while they may strongly affect the measurement of total
(cumulated) daily, monthly or longer time scale rainfall.
On the contrary, systematic mechanical errors related to
the characteristics of the counting of the tips, though scarcely
relevant in terms of cumulated values, may have a large
impact on the measurement of rainfall intensity, with
increasing impact upon increasing the rainfall rate.
Precipitation measurements are
affected by a number of error sources
due to uncertainties in both the
catching and counting phase.
Counting errors
Tipping-Bucket Rain gauge (TBR)
The measurement of rainfall intensity,
traditionally performed by means of tipping
bucket rain gauges is therefore subject to a
systematic underestimation of high rain rates
due to the amount of water lost during the tipping
movement of the bucket.
Although this intrinsic inaccuracy can be suitably
corrected through dynamic calibration of the
gauge, the usual operational practice in many
weather services and manufacturers relies upon
a single point calibration, based on the
assumption that dynamic calibration is not much
significant when the total rainfall depth is to be
recorded.
Such a single point calibration also results in
some overestimation of low intensity rainfall
due to the artificial displacement of the zero error
condition.
The tipping-bucket rain gauge
Single point calibration vs. Dynamic calibration
(about 60 instruments, various models, used at the former Hydrographic Service of Genoa - Italy)
30
Overestimation (??)
100
ar
r
II
I
hn = nominal rain depth per tip
(e.g. 0,2 mm – settings of the data logger)
hv = actual rain depth per tip
hn = hv always underestimation
hn > hv overestimation (move the error curve upward)
hn < hv underestimation (move the error curve downward)
ADJUSTMENT OF THE STOP SCREWS
hv = f(hn) : e% = 0 at I = Irif single point calibration
hvd = hvs ? balancing of the two buckets
Calculation of hn based on the Vn of each bucket and the collector diameter D
20 g = 20000 mm3 (r = 1g/cm3)
1000 cm2 = 100000 mm2 20 g / 1000 cm2 = 0,2 mm
(sensitivity of the instrument)
Single point calibration vs. Dynamic calibration
Dynamic calibration – correction curve
(La B
arb
era
et
al., 2002)
Propagation of the errors – extreme event statistics
DEPTH-DURATION-FREQUENCY CURVES
(La B
arb
era
et
al., 2002)
Propagation of the errors – extreme event statistics
Underestimation of design rainfall: historical records
Recorded historical series
t = ?
≈ 1 min ≈ 1 hour
Direct correction
using a
calibration curve
Single series of corrected data
Statistics of extreme values
Stochastic downscaling
(disaggregation scheme ?)
with MonteCarlo generation
Correction using
a calibration curve
Ensemble of corrected series
Most probable values
Typic
ally
short
record
s (
recent)
T
= 3
0 –
60 y
ears
Typic
ally
long re
cord
s (h
isto
rical)
T =
100
-200 y
ears
(Molin
i et
al.,
2005)
Obtained «gain» from direct correction of the series recorded at a
high resolution (1 min) in Genoa – Villa Cambiaso
Underestimation of design rainfall: historical records
Molini, Lanza e La Barbera (2005).
The impact of TBRs measurement
errors on design rainfall for urban-
scale applications.
Hydrological Processes, 19(5)
Underestimation of design rainfall
Molini et al., 2005a
Based on the requirements for the measurement of
liquid precipitation intensity at the ground
established by the Expert Meeting on Rainfall
Intensity Measurements, Bratislava (Slovak Rep.),
April 2001, the WMO initiated in September 2004
the first
LABORATORY INTERCOMPARISON
OF RAINFALL INTENSITY (RI) GAUGES.
The intercomparison was held at the accredited
laboratories of the Royal Netherlands
Meteorological Institute (KNMI), Météo France,
and the University of Genova – DIAm, in Italy.
Father Francesco Denza (1872) – Italian Meteorological Society
“… in order for meteorological investigations to deliver progresses for the
human beings … it is necessary not only to have numerous observers and
observations/measurements that are taken with intelligence and accuracy, but
also that meteorological investigations are performed with the same
methodology and carefully intercompared instruments”.
DIAM
UNIGE
(Project Leader:
Luca G. Lanza)
The WMO/CIMO “intercomparisons”
WMOWMO
WMO Laboratory Intercomparison
of Rainfall Intensity GaugesWMO Laboratory Intercomparison
of Rainfall Intensity Gauges
WORLD METEOROLOGICAL ORGANIZATION
COMMISSION FOR INSTRUMENTS AND
METHODS OF OBSERVATION
EXPERT TEAM ON SURFACE-BASED
INSTRUMENT INTERCOMPARISONS AND
CALIBRATION METHODS
INTERNATIONAL ORGANIZING COMMITTEE
(IOC) ON SURFACE-BASED
INSTRUMENTS INTERSOMPARISONS
WMOWMO
WMO Laboratory Intercomparison
OBJECTIVES:The main objective of the intercomparison was to
test the performances of catchment type rainfall
intensity gauges of different measuring principles
under documented conditions.
Further objectives can be summarized as follows:
• To define a standardized procedure for
laboratory calibration of catchment type rain
gauges, including uncertainty of laboratory testing
devices within the range from 2 to 2000 mm/h;
• To performances of the instruments under test;
• To comment on the need to proceed with a field
intercomparison of catchment type of rainfall
intensity gauges;
• To identify and recommend the most suitable
method and equipment for reference purposes
within the field intercomparison of catching and
non-catching types of gauges;
• To provide information on different measurement
systems relevant to improving the homogeneity of
rainfall time series with special consideration given
to high rainfall intensities
WMOWMO
WMO Laboratory Intercomparison
List of instruments involved in the
Laboratory Intercomparison WMOWMO
Genoa, DIAm
MeteoFrance
De Bilt, KNMI
DIAM
UNIGE
WMO Laboratory Intercomparison
WMOWMO
WMO Laboratory Intercomparison
Tipping-bucket rain gauges – Dynamic calibration
Each test was performed at least at seven reference
flow rates with the following rules :
• At least at 2, 20, 50, 90, 130, 170, 200 mm/h;
• If the maximum declared intensity is less or equal to
500 mm·h-1, further reference intensities are determined
at 300 and 500 mm·h-1.
• Beyond that, three further reference intensities are
determined logarithmically between 200 mm·h-1 up to
the maximum declared intensity.
The reference intensity is within the following limits:
1.5 – 4 mm·h-1 at 2 mm·h-1
15 – 25 mm·h-1 at 20 mm·h-1
and within a limit of 10% at higher intensities
Water source Water collectorConstant
flow rate
Weighing device Weighing device
Fixed head or pump
Computer control
Rain
gauge
WMOWMO
WMO Laboratory Intercomparison
Weighing gauges
In addition to measurements based on constant flow rates, the step response of each instrument was
checked based on the devices developed by each laboratory.
The step response of the weighing gauges was measured by switching between two different constant flows,
namely from 0 mm·h-1 to 200 mm·h-1 and back to 0 mm·h-1.
The constant flow was applied until the output signal of the weighing rain gauge was stabilized. The time
resolution of the measurement was higher than 1 minute, e.g. 10 seconds, and the possible delay was
evaluated by determining the first time interval when the measure is stabilized, within a maximum period of 10
minutes..
WMOWMO
Tipping-bucket rain gauges
RIMCO
HS-TB3
PAAR
Meteoservis MR3H
SIAP
CAE
ETG
Lambrecht
Casella
Waterlog
Yokogawa
IMD
0
100
200
300
0 100 200 300
I reference [mm/h]
I m
ea
su
red
[m
m/h
]
600 mm/h
720 mm/h
635 mm/h
2000 mm/h
ra II
WMO Laboratory Intercomparison
WMOWMO
Tipping-bucket rain gauges with correction
Meteoservis MR3H
ETG
Yokogawa
0
100
200
300
0 100 200 300
I reference [mm/h]
I m
ea
su
red
[m
m/h
]
CAE
Waterlog
Tipping-bucket rain gauges
RIMCO HS-TB3
PAAR
IMS
SIAP
Lambrecht
Casella
0
100
200
300
0 100 200 300
I reference [mm/h]
I m
ea
su
red
[m
m/h
]
WMO Laboratory Intercomparison
WMOWMO
Water level gauges
0
100
200
300
0 100 200 300
I reference [mm/h]
I m
ea
su
red
[m
m/h
]
Alluvion
Serosi
Weighing gauges
0
100
200
300
0 100 200 300
I reference [mm/h]
I m
ea
su
red
[m
m/h
]
WMO Laboratory Intercomparison
WMOWMO
RIMCO
HS TB-3
MR3H-FC MPS
Serosi
Vaisala
Geonor
IMS
PAAR
SIAP
Casella
Lambrecht
MRW500
YokogawaETG
CAE
OTT
Waterlog
Alluvion
-0.2
-0.15
-0.1
-0.05
0
0.05
0.1
0.02
-0.02
Average relative error
0
50
100
150
200
250
300
0 50 100 150 200 250 300
I reference [mm/h]
I m
easu
red
[m
m/h
]
A r
A m
eAVG = (A m - A r) / A r
Synthetic results of the Intercomparison
WMO Laboratory Intercomparison
WMOWMO
WMO Laboratory Intercomparison
WHAT HAVE WE LEARNED from the INTERCOMPARISON ?
All the investigated instruments are subject to
errors in the measurements of rainfall intensity.
Those tipping-bucket rain gauges that are equipped with a suitable
software correction did provide good results.
Those with no correction show significant errors.
The error of weighing gauges is lower than for tipping-bucket gauges
under constant flow rate conditions, provided the instrument is
stabilizzed, which may take a considerable time (minutes).
However, those instruments show significant delays in detecting variations
in time of the rain intensity.
In many cases significant differences have been noted in the behaviour of
two individuals of the same model. Tests are necessary ona higher
number of individuals (at least 30) to better evaluate the associated
uncertainty.
WMOWMO
Lanza et al. (2005).
Final Report WMO Laboratory Intercomparison of Rainfall Intensity
Gauges; De Bilt (The Netherlands), Genoa (Italy), Trappes (France);
September 2004 – September 2005
(available at http://www.wmo.int/pages/prog/www/IMOP/reports.html)
Prof. V.Vaisala Award 2008
WMOWMO
Laboratory controlled conditions
constant flow rates
known reference intensity
counting errors
Drawbacks:
Rainfall is not real (variability,
intermittency, …)
No catching errors involved
Working conditions are not real
Follow-up in the field
WMO Field Intercomparison of
Rainfall Intensity Gauges
Vigna di Valle (Rome) – OTT07
The main objective of the Laboratory Intercomparison
was to test the performance of rainfall intensity catching
type gauges from various manufacturers
under documented conditions.
From the laboratory to field tests …
WMOWMO
WMO FIELD INTERCOMPARISON of RAINFALL INTENSITY (RI) GAUGES
Università di Genova (DICAT)
Servizio Meteorologico dell’Aeronautica, ReSMA, Vigna di Valle, Roma
2007-2
009
http://w
ww
.dic
at.
unig
e.it/
wm
o WMOWMO
1871 – Symons performs the first
intercomparison of rain gauges
at Hawskers (Yokshire)
Experiment for studying the
effect of installation hight of
the instrument (Symons 1862)
A long tradition of Field Intercomparisons existed …
RESMA – Experimentation Centre for
Meteorological Instruments and historic observatory
1911
ACTIVITIES:
• Intercomparisons of meteorological instruments
• Performance tests and monitoring for WMO-GAW
• Metrological aspects of the measurement: reference standards and uncertainty
The Field Test site in Vigna di Valle
WMO Intercomparison in the Field
WMOWMO
Preliminarylaboratory tests
WMO Intercomparison in the Field
WMOWMO
WMO Intercomparison in the Field
WMOWMO
WMO Intercomparison in the Field
WMOWMO
WMO Intercomparison in the Field
0
100
50
0
10050 75
75
Iref [mm h-1
]
Imeas [mm h-1
]
200100
300
250150
100
150
200
250
300 Iref [mm h
-1]
Imeas [mm h-1
]
Range: 0-100 mm·h-1 Range: 100-300 mm·h-1
ALL CATCHING TYPE GAUGES
Note: both axes of the graphs are rescaled using a third order power law
WMOWMO
WMO Intercomparison in the Field
Range: 0-100 mm·h-1 Range: 100-300 mm·h-1
ALL THE TIPPING-BUCKET RAIN GAUGES
0
100
50
0
10050 75
75
Iref [mm h-1
]
Imeas [mm h-1
]
200100
300
250150
100
150
200
250
300
Iref [mm h-1
]
Imeas [mm h-1
]
Note: both axes of the graphs are rescaled using a third order power law
WMOWMO
WMO Intercomparison in the Field
Range: 0-100 mm·h-1 Range: 100-300 mm·h-1
TIPPING-BUCKET RAIN GAUGES WITH
CORRECTION APPLIED
0
100
50
0
10050 75
75
Iref [mm h-1
]
Imeas [mm h-1
]
200100
300
250150
100
150
200
250
300
Iref [mm h-1
]
Imeas [mm h-1
]
Note: both axes of the graphs are rescaled using a third order power law
WMOWMO
WMO Intercomparison in the Field
Range: 0-100 mm·h-1 Range: 100-300 mm·h-1
WEIGHING GAUGES
0
100
50
0
10050 75
75
Iref [mm h-1
]
Imeas [mm h-1
]
200100
300
250150
100
150
200
250
300
Iref [mm h-1
]
Imeas [mm h-1
]
Note: both axes of the graphs are rescaled using a third order power law
WMOWMO
Variability of the resultsat 1 minute resolution
TB
Rw
ith c
orr
ection
WMO Intercomparison in the Field
WMOWMO
Variability of the resultsat 1 minute resolution
WMO Intercomparison in the Field
TBR with no correction
WMOWMO
Variability of the resultsat 1 minute resolution
WMO Intercomparison in the Field
WG good dynamic response
WMOWMO
Variability of the resultsat 1 minute resolution
WMO Intercomparison in the Field
WG scarce dynamic response
WMOWMO
WMO Intercomparison in the Field
• Avoids perturbation of the air flow
at the instrument’s collector – wind
effects (JEVONS, 1861)
• Eliminates the influence of the
shape of the gauge on the air flow
• The influence of the necessary
collector on the surrounding air flow
is riduced to a minimum because the
surface of the collector is placed in
the air layer with the minimum air
movement.
• Also the influence of the turbulent
vertical movements is reduced to a
minimum, because these vanish in
the vicinity of the ground.
The reference (standard) “pit gauge"WMOWMO
According to EN13798:2002
The field test site
WMO Intercomparison in the Field
WMOWMO
WMO Intercomparison in the Field
At the University of Genova a portable device
was developed with the aim of performing in
situ the same kind of tests that have been
preliminary performed for the calibration of
all catching type instruments in controlled
laboratory conditions.
WMOWMO
WMO Intercomparison in the Field
a WMO procedure was defined for performing tests in
the field about the instrument calibration …
Brevetto n° 102006A000868 del 07/12/2006
Stagi L. and Lanza, L.G. (2006). Device for the
generation of various known and constant liquid
flow rates. Patent University of Genoa
n. 102006A000868, 7 December 2006WMOWMO
WMO Intercomparison in the Field
WMOWMO
WMO Intercomparison in the Field
WMOWMO
WMO Intercomparison in the Field
WMOWMO
WMO Intercomparison in the Field
WMOWMO
WMO Intercomparison in the Field
WMOWMO
0
20
40
60
80
100
120
140
160
180
200
220
0 20 40 60 80 100 120 140 160 180 200 220
Reference RI [mm•h-1
]
Measured RI [mm•h-1
]
RIM7499020 Mc VAN
Rain Collector II-DAVIS
DQA031-LSI LASTEM
PP040-MTX
AP23 PAAR
ARG100-EML
TIPPING-BUCKET (with no correction)
WMO Intercomparison in the Field
WMOWMO
0
20
40
60
80
100
120
140
160
180
200
220
0 20 40 60 80 100 120 140 160 180 200 220
Reference RI [mm•h-1
]
Measured RI [mm•h-1
]
PMB2-CAE
UMB7525/I/SIAP-MICROS
R102-ETG
TIPPING-BUCKET (with software correction)
WMO Intercomparison in the Field
WMOWMO
0
20
40
60
80
100
120
140
160
180
200
220
0 20 40 60 80 100 120 140 160 180 200 220
Reference RI [mm•h-1
]
Measured RI [mm•h-1
]LB-15188-LAMBRECHT
PT 5.4032.35.008-THIES
TIPPING-BUCKET (with pulse correction)
WMO Intercomparison in the Field
WMOWMO
0
20
40
60
80
100
120
140
160
180
200
220
0 20 40 60 80 100 120 140 160 180 200 220
Reference RI [mm•h-1
]
Measured RI [mm•h-1
]PG200-EWS
T200B-
PLUVIO-OTT
MRW500 -METEOSERVIS
TRwS-MPS
VRG101-VAISALA
ANS 410/H-EIGENBRODT
WEGHING GAUGES
WMO Intercomparison in the Field
WMOWMO
0
20
40
60
80
100
120
140
160
180
200
220
0 20 40 60 80 100 120 140 160 180 200 220
Reference RI [mm•h-1
]
Measured RI [mm•h-1
]
PWD22-VAISALA
LCR-PVK
ATTEX
WXT510-VAISALA
PARSIVEL-OTT
LPM-THIES
NON CATCHING TYPE GAUGES
WMO Intercomparison in the Field
WMOWMO
Vuerich, E., Monesi, C., Lanza, L.G., Stagi, L. and E. Lanzinger (2009).
WMO Field Intercomparison of Rainfall Intensity Gauges. World
Meteorological Organisation – Instruments and Observing Methods Rep.
No. 99, WMO/TD No. 1504, pp. 286
(available at http://www.wmo.int/pages/prog/www/IMOP/reports.html)
Prof. V.Vaisala Award 2010
WMOWMO
• La Barbera, P., L.G. Lanza and L. Stagi (2002). Influence of systematic mechanical errors of tipping-
bucket rain gauges on the statistics of rainfall extremes. Water Sci. Techn., 45(2), 1-9.
• Molini, A., Lanza, L.G. e P. La Barbera (2005). The impact of tipping bucket measurement errors on
design rainfall for urban-scale applications. Hydrological Processes, 19(5), 1073-1088.
• Molini, A., Lanza, L.G. e P. La Barbera (2005). Improving the accuracy of rain intensity records by
disaggregation techniques. Atmos. Res., 77, 203-217.
• Lanza, L., Leroy, M., Alexandropoulos, C., Stagi, L. and Wauben, W. (2005). Laboratory Intercomparison
of Rainfall Intensity Gauges. World Meteorological Organisation – Instruments and Observing Methods
Rep. No. 84, WMO/TD No. 1304.
• Lanza, L.G. and L. Stagi (2008). Certified accuracy of rainfall data as a standard requirement in scientific
investigations. Advances in Geosciences, 16, 43-48.
• Lanza, L.G. and E. Vuerich (2009). The WMO Field Intercomparison of Rain Intensity Gauges. Amos.
Res., 94, 534-543.
• Lanza, L.G. and L. Stagi (2009). High resolution performances of catching type rain gauges from the
laboratory phase of the WMO Field Intercomparison of Rain Intensity Gauges. Atmos. Res., 94, 555-563.
• Lanza, L.G., Vuerich, E. and I. Gnecco (2010). Analysis of highly accurate rain intensity measurements
from a field test site. Advances in Geosciences, 25, 37-44.
• Lanza, L.G. e Vuerich, E. (2012). Non-parametric analysis of deviations of one-minute rain intensity
measurements from the WMO field intercomparison. Atmos. Res., 103, 52-59.
• Lanza, L.G. e Stagi, L. (2012). Non-parametric error distribution analysis from the laboratory calibration of
various rainfall intensity gauges. Water Sci. Tecn., 65(10), 1745-1752.
References
http://www.precipitation-intensity.it
for further information:
References
• Colli, M., Lanza, L.G. and P.W. Chan (2013). Co-located tipping-bucket and optical drop counter RI
measurements and a simulated correction algorithm. Atmos. Res., 119, 3-12.
• Colli, M., Lanza, L.G. and P. La Barbera (2013). Performance of a weighing rain gauge under laboratory
simulated time-varying reference rainfall rates, Atmos. Res., 131, 3-12
• Colli, M., Lanza, L.G., La Barbera, P. and P.W. Chan (2014). Measurement accuracy of weighing and
tipping-bucket rainfall intensity gauges under dynamic laboratory testing. Atmos. Res., 144, 186-194.
• Santana, M.A.A., Guimarães, P.L.O., Lanza, L.G. and E. Vuerich (2015). Metrological analysis of a
gravimetric calibration system for tipping-bucket rain gauges. Meteorol. Appl., 22, 879-885.
• Stagnaro, M., Colli, M., Lanza, L.G. and P.W. Chan (2016). Performance of post-processing algorithms for
rainfall intensity measurements of tipping-bucket rain gauges. J. Atmos. Meas. Techn., 9, 5699–5706.
Wrap-up and perspectives
Generally, precipitation gauges (all types) are not satisfactorily calibrated.
Rainfall and snowfall are still widely measured today with much lower accuracy
than the present knowledge and technology would actually permit.
Common measurement procedures :
- do not correct/adjust for SYSTEMATIC BIASES
- do not report the measurement UNCERTAINTY
TRACEABILITY of the measurement
to the international standards can not be guaranteed
Before TRACEABILITY can be correctly addressed we need:
1) BIAS assessment and correction/adjustment
for catching type instruments:
a) dynamic calibration in the laboratory
b) Interpretation and correction algorithms
TBRs time-of-tip algorithms and correction for SME
WGs time constant assessment & step response correction
Drop counters drop volume calibration and correction
(…)
c) correction for wind-induced undercatch
d) compliance with WMO, CEN, … ISO ?
NO replacement of the existing instruments
is generally required to achieve all of the above
Wrap-up and perspectives
2) UNCERTAINTY assessment
for catching type instruments:
a) Calibration uncertainty from dynamic calibration
b) Uncertainty sources in the field from intercomparison campaigns
(WMO FI/RI, SPICE, …)
against a suitable reference
e,g, a future regional intercomparison of Rainfall Intensity gauges in Region II & V
(in collaboration with the WMO/CIMO Lead Centre on Precipitation Intensity)
For the CALIBRATION
of non-catching type instruments (stay tuned …)
Wrap-up and perspectives